The present invention generally relates to composite materials and their related processes. More particularly, this invention is directed to processes of forming a three-dimensional textile preform whose structure can be more readily densified and uniformly infiltrated to yield a dense composite component.
Ceramic matrix composite (CMC) materials generally comprise a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material serves as the load-bearing constituent of the CMC in the event of a matrix crack, while the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. Of particular interest to high-temperature applications are silicon-based composites, such as silicon carbide (SiC) as the matrix and/or reinforcement material. As examples, SiC fibers (filaments) and tows (bundles of filaments) have been used as a reinforcement material for a variety of ceramic matrix materials, including SiC, TiC, Si3N4, and Al2O3.
Continuous fiber reinforced ceramic composite (CFCC) materials are a type of CMC that offers light weight, high strength, and high stiffness for a variety of high temperature load-bearing applications. A CFCC material is generally characterized by continuous fibers that may be arranged to form a unidirectional array of fibers, or bundled in tows that are arranged to form a unidirectional array of tows, or bundled in tows that are woven to form a two-dimensional fabric or woven, braided, etc., to form a three-dimensional fabric. Conventional textile patterns can be used to form a textile preform in which two or more sets of tows are interlaced. The terms “warp,” “weft,” and “bias” are commonly used to identify the orientation of tows relative to weaving processes, and the terms “axial” and “braider” are commonly used to identify the orientation of tows relative to braiding processes. Warp and axial tows are those that, during the fabrication of a preform, continuously pass through a weaving or braiding machine so as to be parallel to the process direction of the preform. Weft (or fill) and bias tows run transverse (perpendicular and oblique, respectively) to warp tows of a woven preform, and braider tows run transverse to the axial tows of a braided preform. Because weft, bias, and braider tows are interwoven with the warp and axial tows, the former group may be termed dynamic tows and the latter static or stationary tows in reference to the weaving and braiding processes. Because the dynamic tows are interlaced with the static tows, the static tows tend to be substantially straight. The individual tows may be coated with a debond interface, such as boron nitride (BN) or carbon, forming a weak interface coating that allows for debonding and matrix crack deflection between the tows and the ceramic matrix material. As cracks develop in the CMC, one or more fibers bridging the crack act to redistribute the load to adjacent fibers and regions of the matrix material, thus inhibiting or at least slowing further propagation of the crack.
CMC components having complex shapes and those subject to high mechanical and thermal loads typically require a tailored three-dimensional fiber preform architecture that is densified with a ceramic matrix material, such as by infiltrating the preform with the desired matrix material (or a precursor thereof) to fill the porosity within the preform. Conventional three-dimensional preform fabrication processes (such as braiding, weaving, etc.) utilize dry or lightly sized tow whose brittle ceramic filaments can suffer damage from the fabrication process. For those CMC components requiring preforms with large tow size (high filament counts) to obtain desired part dimensions, three-dimensional preform fabrication processes do not allow for direct control of “filament packing” within the tows or shaping of the tow cross-sections to obtain optimized fiber weighting. The requirement or use of large tow sizes, in conjunction with the abrasive nature of the textile preforming process, can lead to filament breakage during preform fabrication and difficulties in matrix infiltration during composite densification, both of which can negatively affect the mechanical and physical properties of the CMC. To compensate for this, state of the art preform fabrication methods often employ both a lightly sized tow to reduce filament breakage during preforming, and an arbitrarily set limit on size of tow or filament count to aid in proper matrix infiltration. Prior CMC work has indicated that a key to obtaining good fiber coating and composite properties is related to spreading of the fibers inside the tows of two-dimensional fabrics used to form a CMC preform. For two-dimensional fabrics, this has been achieved by using mechanical and ultrasonic fluffing techniques. However, such techniques are not effective for three-dimensional fabrics.
In view of the above, it would be desirable if a process were available by which tow shape and filament packing within tows can be more readily predetermined and controlled while also providing for protection of the tow filaments during the preform fabrication process.